It’s a fitting description for the peculiar galaxy pictured right. Consisting of a bright yellow spherical core surrounded by a symmetrical luminous ring of blue-tinged stars, Hoag’s object looks like it’s doing an impression of the planet Saturn. This is unlike any other galaxy – and so perplexing to astronomers that it might as well exist in another universe.

“It’s one of these weird little objects you point at without fully understanding what they mean,” explains François Schweizer of the Carnegie Observatories in Pasadena, California.

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He has done his bit in a 60-year struggle to pin down the forces and events that might have created the enigmatic galaxy. Recent observations suggest the core came first and the ring added in later – but the details are still puzzling.

Funhouse mirror

Galaxies come in a variety of shapes, but most fit into one of just a few categories – simple spheroids or ellipses, flattened discs, elegant spirals like the Milky Way or irregularly shaped blobs. But Hoag’s object is so bizarre that when astronomer Arthur Hoag first spotted it in 1950, he wasn’t even sure it was a galaxy.

Instead he thought it might be a ring-shaped puff of gas given off by a dying star&colon; a type of object called a planetary nebula that is commonly found in the Milky Way. But Hoag himself was unsatisfied with that explanation, noting that the ring was not emitting light at the wavelengths characteristic of the hot gas in such clouds.

He further speculated that the ring could be an optical illusion, the result of a phenomenon called gravitational lensing, in which a foreground galaxy’s gravity bends the light of a more distant galaxy, giving it a peculiar shape – like looking at it in a funhouse mirror.

But that explanation failed, too. In 1974, observations of the core of Hoag’s object showed it weighs far too little to cause the extreme gravitational lensing needed to turn a background galaxy into a ring.

One way to test this scenario is to look for faint galaxy fragments around Hoag’s object, since these are often seen in the aftermath of a galaxy torn asunder. But the most sensitive observations yet, made with a 6-metre telescope in Russia, have found nothing of the kind, says a team led by Ido Finkelman of Tel Aviv University in Israel in a study to appear in Monthly Notices of the Royal Astronomical Society.

That makes the team sceptical of the galactic cannibalism scenario, though they admit that if the carnage happened more than 3 billion years ago, there might not be any detritus left to see.

Wild optimism

As an alternative, they suggest the core of Hoag’s object simply sucked in gas floating in intergalactic space via gravity to build up the supply needed to form the ring.

It’s a reasonable idea, says Schweizer, who was not involved in the new study. But he is not ready to rule out an ancient act of cannibalism.

Further observations are needed to point the way to the answer&colon; “Perhaps we haven’t even yet thought of the correct one,” he suggests.

Hoag’s object is a surprisingly difficult nut to crack. The first report on it by Hoag himself seems wildly optimistic in hindsight. “Since the object is unique, I suggest that a proper identification would be a worthwhile short-term project,” he wrote.